The invention relates to a centralized monitoring method for a security system in which abnormal conditions such as the occurrence of a fire or gas leakage are detected in a centralized manner.
In the case where a number of terminal units, each having an element such as a fire sensor or a gas leakage detector, are connected to lines which are connected to a central unit thereby to detect the abnormal conditions in the area where the terminal units are installed, a polling system, as disclosed by Japanese Laid-Open patent application No. 103874/1980 has been employed. In this system, addresses are assigned to the terminal units, and the central unit calls the terminal units in sequence using these addresses so that only the terminal units thus called transmit their data, for instance, fire detection signals, to the central unit.
In the above-described polling system, no terminal unit can transmit its data to the central unit until it is called by the central unit. This calling time interval increases with the number of terminal units connected to the lines. Therefore, if the terminal units include ones which must transmit data to the central unit within a predetermined period of time after the formation of the data, then the number of terminal units which can be connected to the lines is limited.
An example of a terminal unit which must transmit data quickly is a transmitter in a fire alarm system. The transmitter displays, when a person who has found a fire operates a push button or the like to inform of the fire to the central unit, a response signal representing the fact that the receiver has received the information. However, in the polling system, it may happen that the button is pushed for a time interval of less than the polling period, that is, the time needed between transmission of the information and the reception of the response signal is longer than the time the push button is held. The person who attempted to report the fire may wonder why there is such a delay and give up in the attempt to report the fire. Thus, this delay time should be decreased.
If transmitters operating as described above are employed in a centralized monitoring method employing a polling system, it is essential to sufficiently take the number of terminal units to be connected to one line into consideration.